Mobile telephones have drastically developed during the past decade so that in the near future, the most developed telephones will provide cellular, global position system (GPS) and low power radio frequency (LPRF) radio (e.g., wireless local area network (WLAN), Bluetooth, ultrawideband (UWB), radio frequency identification (RFID), etc.) communications all in the same portable device. Typically, these devices are designed to be hand held, but other form factors such as wristwatch type and wearable devices may also emerge. Conventionally, such devices typically include two or more antennas to support the different types of wireless communications. In addition, many developed telephones will provide for antenna diversity in one or more of these communication techniques by including multiple antennas that provide communication in accordance with the same type of communication.
An antenna radiates electromagnetic waves with a power that is a function of its electric feed signal's power and frequency. An antenna has a resonant frequency at which it has the highest gain, the gain often referred to as the radiation power. The highest radiation power not only affects the transmission efficiency but also the reception efficiency so that an antenna is also most sensitive to receive radio signals at its resonant frequency or frequencies. Hence, an antenna best absorbs radio signals at its resonant frequency.
With two or more different antennas used for different radio communications such as GPS and LPRF, for instance, the frequency bands on which these antennas operate are very close to each other or overlap, because many new radio standards share the frequency bands around 1.5-2.4 GHz region. The antennas are bound to reside close to each other if the entire apparatus housing them is small, perhaps a few centimeters in maximum dimension, and hence the coupling between the antennas is also bound to increase. Generally, coupling between antennas is a condition when a portion of the radio signals transmitted by one antenna are captured by another antenna. Typically, as the coupling between antennas increases, so does the interference between the radio receivers and transmitters. Thus, it becomes more difficult to filter the undesired interference from the other transmitter. It is thus necessary to ensure a sufficient level of isolation to provide satisfactory efficiency for the transmissions.
As will be appreciated, coupling not only takes place when two different antennas are used in proximity to each other, but the mere existence of the second antenna will often draw some radio power. In this regard, the radio power draw increases the closer the antennas are placed to one another, and the closer their resonant frequencies. Thus, the isolation has often been enhanced by locating different antennas as far from each other as possible, such as by using different polarizations, manually removing an unused antenna from the device for periods when the unused antenna is not needed, placing radiation obstacles between the antennas, and/or disconnecting the ground or feed of unused antennas. And whereas designing wireless communication devices to include separate antennas is adequate for providing communication in accordance with different types of wireless communications, it is always desirable to improve upon such designs. In this regard, due to portability requirements, the size of the radio device should be kept to a bare minimum while maintaining isolation for communicating in accordance with different wireless communications techniques.